The invention relates to an interferometer as set forth in the preamble of the appended independent claim.
Many different interferometers are available. The simplest interferometer is a Michelson plane mirror interferometer, whose main components are a beamsplitter, a fixed mirror, and a movable mirror. A light beam hits the beamsplitter, whereupon part of the light beam passes through, reflects from the fixed mirror back to the beamsplitter and therefrom to a receiver, which can be, for example, a photocell or a human eye. Part of the light beam reflects from the beamsplitter to the movable mirror, from which it reflects back to the beamsplitter and further to the receiver. The beams incident on the receiver from the fixed and movable mirror interfere. If the distance from both mirrors to the beamsplitter is exactly equal, said distances include the same number of wavelengths of the applied light. If the movable mirror is moved closer to or further away from the beamsplitter, the receiver can register interference maxima the distance of which is half of the wavelength.
An interferometer is used e.g. for determining distances at a very high accuracy, for mapping roughnesses in various surfaces, as well as for determining a wavelength or wavelengths (spectra) in electromagnetic radiation.
The widest application range for interferometers is spectrometry. An important feature in spectrometric applications is the capability of moving a movable mirror at high precision without tilting the mirror. High precision motion has been pursued by developing so-called carousel interferometers, wherein the changing of optical path differences is performed in such a way that a carousel, constituted by pairs of mirrors mounted on a stationary structure, is rotated back and forth around an axis. Hence, this increases the optical path for one beam and decreases it for the other.
For example, patent publication U.S. Pat. No. 5,159,405 discloses an interferometer comprising a stationary beamsplitter, a rotatable pair of mirrors constituted by two plane mirrors, as well as a pair of mirrors constituted by two plane mirrors set at an angle relative to each other and returning the beams. A problem in the cited solution is its instability. Although the cited solution represents an implement with respect to prior art interferometers, such an instrument still has obvious faults. The principal reason for the inaccuracy of an interferometer as disclosed in the publication U.S. Pat. No. 5,159,405 is a long distance between the beamsplitter and the pair of mirrors returning the beams. Thus, when the base plate of an interferometer is subjected to deformation due to pressure and temperature variations, such that one edge thereof expands or contracts more than the other edge, or when the plate is subjected to torsional forces bending the opposite corners of the plate in different directions, the resulting measurement disturbances will be significant.
Patent publication U.S. Pat. No. 5,309,217 discloses a Fourier spectrometer comprising a beamsplitter, a rotatable pair of mirrors constituted by two cube-corner mirrors, as well as two fixed returning mirrors. The cited solution provides a compact configuration. However, a problem in the solution is the use of a cube-corner mirror consisting of three mirrors. In order to render the solution stable and fully functional in all circumstances, the angles between all mirrors included in cube-corner mirrors should be precisely 90°. If the angles deviate slightly from 90°, there will be six images, and a plane wave in the interferometer will be divided into six unequal-phase zones. The manufacture of a cube-corner mirror with all of its angles being precisely 90° is highly expensive, so the manufacturing costs for an instrument as disclosed in the cited publication are extremely high.
The earlier patent publication U.S. Pat. No. 6,075,598 of the applicant discloses a further developed carousel interferometer, wherein a beam reflected from a radiation source is divided by a beamsplitter into two individual beams. In addition, the interferometer described in U.S. Pat. No. 6,075,598 comprises one plane mirror for returning the beams, as well as two pairs of mirrors for reflecting the beamsplitter-emitted beams to said returning plane mirror and the beams returning therefrom further back to the beamsplitter. Said pairs of mirrors guiding the beams between the beamsplitter and the returning plane mirror are mounted on a rigid structure, a carousel, which is adapted to be rotatable around an axis. The solution disclosed in the publication U.S. Pat. No. 6,075,598 provides a highly stable structure, wherein the passage of beams cannot be affected by potential base plate deformations.
It has now been discovered that the solution disclosed in the publication U.S. Pat. No. 6,075,598 can be improved even further. In the solution set forth in U.S. Pat. No. 6,075,598 the mounting of a carousel to effect its rotation in an exactly horizontal position is difficult in some applications. In other words, it has been discovered that, in certain conditions, the rotation axis of a carousel has a tendency towards a slight tilt deviating from a precise vertical plane. If the axis and a carousel mounted thereon become tilted in the direction of a plane of a beamsplitter, the beams will be deflected, such that the end mirror will be hit by the latter either higher or lower and so will the beamsplitter by returning beams. In this case, the deflection of both beams occurs in the same direction, with no disturbance in the operation of the interferometer. However, if the axis and a carousel mounted thereon become tilted in a direction deviating from that of a plane of a beamsplitter, the deflections of beams occur in deviating directions and the operation of the interferometer may be disturbed.
Another present discovery is that one problem in the solution disclosed in U.S. Pat. No. 6,075,598 is its adjustment, which is performed by adjusting the beamsplitter. It has now become evident that the adjustment of a beamsplitter, i.e. the precise setting of its attitude and position, is practically difficult in some cases. One reason for the difficulty is e.g. that the beamsplitter is manufactured in such a soft material that it may deform or yield in the process of performing an adjustment with set screws.
Hence, it is an object of an interferometer of the present invention to obviate or at least alleviate problems caused by the above-stated prior art.
Another object of an interferometer of the present invention is to provide an accurate and highly stable interferometer, wherein the effect of potential trouble sources on measuring accuracy is eliminated or at least undermined.
A still further object of the present invention is to provide an interferometer, which is readily and precisely adjustable.
Still another object of an interferometer of the present invention is to provide a compact interferometer, which is capable of achieving a considerable change in an optical path difference with respect to external interferometer dimensions.